Conventional 2-bed PSA systems have feed adsorption and waste removal steps, and as such subsequent cycling of the adsorbers is inevitable. During such cycling processes, adsorption fronts are created within the adsorbent beds. Efficient cycle design with advanced adsorbents has resulted in moving these adsorption fronts close to breakthrough at both ends of the bed. This results in a gas enriched in the light component being left at the top of the adsorption front and also in any upper head space within the adsorber vessel at the end of a cycle. In a PSA cycle this enriched gas is referred to as void gas.
Conventional processes either leave this non-recovered void gas in the adsorber, which will act as an inefficient purge during the evacuation period, or remove this void gas prior to the waste removal step by driving the separation front out of the top of the adsorber during the product make step or equalization depressurization steps. These options result in inefficient operation of the cycle.
Improvements to conventional processes have included the addition of equalization, purge and product repressurization steps. A typical process and system is shown in FIG. 1 and described in U.S. Pat. No. 5,702,504 (Smolarek et al). This process requires two adsorber beds, one product receiver (PT), one feed blower and one vacuum pump with appropriate switching valves. The steps in the process are as follows:
Step #1 Raising pressure feed with counter-current top pressurization with product tank oxygen; PA1 Step #2 Raising pressure feed; PA1 Step #3 Constant pressure product make step; PA1 Step #4 Co-current void gas recovery supplying this gas as equalization to the other adsorber bed; PA1 Step #5 Co-current void gas recovery with counter-current evacuation; supplying void gas to the other adsorber bed; PA1 Step #6 Falling pressure evacuation; PA1 Step #7 Falling pressure evacuation; PA1 Step #8 Counter-current purge with product oxygen; PA1 Step #9 Counter-current purge/repressurization with void gas from the other bed; PA1 Step #10 Counter-current repressurization with void gas from the other bed while feeding. PA1 a) pressurizing said adsorbent bed to a high pressure with a feed of said mixture to enable said adsorbent bed to adsorb said less preferred gas while simultaneously counter-currently feeding gas obtained from a product tank containing more preferred gas; PA1 b) extracting from said bed at said high bed pressure, a flow of said more preferred gas and storing at least some of said flow of more preferred gas in a product tank; PA1 c) desorbing said less preferred gas from said adsorbent bed by feeding void gas in said enclosure to a void gas storage tank, while simultaneously desorbing said less preferred gas from said adsorbent bed by venting said adsorbent bed to a low pressure region; PA1 d) terminating feeding of said void gas to said void gas storage tank; PA1 e) further desorbing said less preferred gas from said adsorbent bed by venting said adsorbent bed to a low pressure region; PA1 f) purging said adsorbent bed by feeding to said adsorbent bed a portion of said void gas from said void gas storage tank while venting said adsorbent bed; and, PA1 g) pressurizing said adsorbent bed to an intermediate pressure with a flow of equalization gas from a second adsorber bed; and repeating steps a-g until a requirement for said more preferred gas is satisfied. PA1 a) pressurizing one of said first adsorbent bed or said second adsorbent bed to a high pressure with a feed of said mixture to enable said adsorbent bed to adsorb said less preferred gas, while simultaneously counter-currently feeding gas obtained from a product tank containing more preferred gas; PA1 b) extracting from said one of said first adsorbent bed or said second adsorbent bed at bed pressure, a flow of said more preferred gas and storing at least some of said flow of more preferred gas in said product tank; PA1 c) desorbing said less preferred gas from said one of said first adsorbent bed or said second adsorbent bed by feeding void gas in said enclosure to a void gas storage tank, while simultaneously desorbing said less preferred gas from said adsorbent bed by venting said adsorbent bed to a low pressure region; PA1 d) terminating feeding of said void gas to said void gas storage tank; PA1 e) further desorbing said less preferred gas from said one of said first adsorbent bed or said second adsorbent bed by venting said one of said first adsorbent bed or said second adsorbent bed to a low pressure region; PA1 f) purging said adsorbent bed means by feeding to said one of said first adsorbent bed or said second adsorbent bed a portion of said void gas from said void gas storage tank while venting said first adsorbent bed or said second adsorbent bed; and PA1 g) pressurizing said one of said first adsorbent bed or said second adsorbent bed to an intermediate pressure with a further flow of equalization gas from one of said first adsorbent bed or said second adsorbent; and
In the cycle described above, top void gas is partially recovered through overlap equalization steps. Unfortunately, the problem with equalization step recovery is that void gas recovery is ended after the beds are equalized. Multiple bed cycles can be employed increasing the void gas recovery, but limits are still reached as to the quantity which can be recovered.
Present cycles, such as those described in U.S. Pat. No. 5,518,526 (Baksh et al) and U.S. Pat. No. 5,702,504 also employ overlap equalization with the feed or evacuation steps. These steps are directed toward increasing the utilization of the adsorbent and mechanical equipment.
These steps do not necessarily increase the recovery of void gas, and in some cases this overlap feature actually reduces the recovery of void gas.
Thus there is a need for PSA processes and systems whereby top void gas may be effectively utilized.